To ensure a high availability and operating security of the electric power supply and to guarantee personal safety in the area of electric installations, power supply networks are being increasingly employed whose active components are separated from the earth potential. In this type of power supply network, called unearthed IT system, an active conductor can have an insulation fault without the running operation having to be interrupted since due to the ideally infinitely high impedance value between the conductor and earth in this first fault case no closed circuit can form. In this context, a faulty state of the IT system, in particular a fault to frame or an earth fault, meaning a conductive connection of inactive system parts to conductive operating parts or a conducting connection of an active conductor to the ground, are to be understood as an insulation fault.
It becomes clear from this point of view that the resistance in the network to be monitored, including all resistances of all connected operating parts to earth (insulation resistance), have to be monitored constantly because a possible further fault on another active conductor (second fault) could cause a fault loop and the fault current running therein in connection with an overcurrent protection circuit would result in a shut-down of the system. Through a constant insulation monitoring of the unearthed IT system a drop in the insulation resistance can be detected and reported in time.
According to the state of the art, the measuring processes for determining the insulation resistance are based in principle on the superposition of a measuring voltage, generated in an insulation monitoring device, between the conductors of the IT network and the ground so that a specific measuring current proportional to the insulation fault occurs, which causes a corresponding voltage drop on a measuring resistance of the insulation monitoring device. If the voltage drop exceeds a certain value as a result of a dropped insulation resistance and thus of a higher measuring current, a report is triggered. In order to prevent measurement distortions with regard to a reliable insulation monitoring in today's modern networks, in which a plurality of operating parts are equipped with electronic components, the distortions being caused for example by direct current components generated by inverters, the measuring methods have been continuously developed further. In pure alternating current networks without distorting direct current components, the method of superimposing a measuring DC voltage can be applied, whereas in faulty environments a controlled, specifically clocked measuring voltage for driving pulsed signals is employed.
When a drop of the insulation resistance has been detected, insulation fault location begins in that the insulation monitoring device or a separate testing device generates a test current and supplies it to the IT network. To be able to reliably detect insulation faults in a DC network and due to the available measuring technology, the test current supplied for insulation fault location preferably also has a pulse-shaped flow of alternating polarity (pulsed current) so that the test current flows through both conductors (L+ and L−) in an alternating manner. This test current signal is detected by all measuring current transformers which lie in the faulty cable outlet of the network and is analyzed and reported by an insulation fault analyzing device. By means of the allocation of measuring current transformer/circuit, respectively cable outlet, the fault position can be located.
In this approach common according to the state of the art it proves disadvantageous that the determination of the insulation resistance and the localization of insulation faults present two separate, self-contained processes. As a result, for example for making a new measurement of the insulation resistance during the already initiated fault search, the supplying of the test pulses has to be interrupted in order to be able to perform the insulation resistance measurement with a suitable measuring current. Thus, a simultaneous determination of the insulation resistance during the fault search is not possible in a simple manner.
Furthermore, devices according to the state of the art do offer the possibility to stabilize the test current in the fault search during the duration of the pulse by means of corresponding circuit devices, such as current regulators and to limit the test current to one or several maximum values, but these configurations only constitute a relatively crude adjustment of the pulsed current to the network conditions so that often an unnecessarily high pulsed current is generated. In the course of this, an excessive heat build-up can occur in the generator circuit, which so far was only counteracted by the use of large-scale cooling bodies or excess temperature deactivations. In a disadvantageous manner these countermeasures thus lead directly or indirectly to an increase in costs due to operation disruptions.
Furthermore, in shut-down IT systems which have to be inspected no voltage is available to drive the pulsed current. Here, consequently, the problem of a suitable voltage supply for generating the test current presents itself as a matter of principle.